Hi Malya,
I have been working as a church pianist since the age of 16. I'm now 27 with 11 years experience in various churches, some of which have very contrasting styles and approaches to worship and organized music. I have encountered many interesting, difficult and amusing situations.
The first piece of advice I will offer is this: the church is a place of worship, and at all times it is important to all musicians and organizers to try and work in harmony, as a team. This is often very difficult because there is such a variety of people coming together, and a lot of times there is a wide spectrum of aesthetic perceptions and technical abilities. When there is a sense of friction or regret from one member to the other, perhaps over a matter such as tempo or style, the entire reason for being there to worship and focus on spiritual matters is counteracted.
Therefore, my first piece of advice would be to make it your priority to preserve the sense of unity and harmony among the musicians. I'm sure you could take the pianist aside and advise him/her to play in a more suitable tempo because, as a singer you can say from experience, the congregation need more opportunity to breathe between phrases. I think the pianist will take advice better if you begin by complimenting him/ her on some aspect of their playing. Perhaps something like: "I've always enjoyed the wonderful lyrical qualities of your playing. Have you ever considered that playing slower may allow the rich harmony you play to resonate more?" or.... "Perhaps playing slower will help the congregation to meditate on the meaning of the text?- such beautiful lyrics!". This is only a suggestion -i'm sure you will know what to say to suit the context of the situation you have there. I have found that sharing positive compliments is the best way to prepare the person for the sensitive topic that you really feel the need to discuss.
Can I ask why you have chosen not to play the piano as you lead in singing? If you feel comfortable with that, or if you feel you could work on this aspect of your own performance, then this could be a fairly attainable solution. You are an experienced singer and therefore you are probably more sensitive to the needs of the congregation, in terms of breathing and clarity of diction, than the current pianist is.
Often praise teams play too fast and I think this is mainly down to lack of experience or insensitivity (sometimes nerves) or a feeling of EGO often associated with being exposed as a musician. Perhaps you could call a mid-week meeting, for all the people elected to contribute, and as supervisor of the music you could remind everyone of the fundamentals and present general advice to the group without having the need to single out the piano player. Perhaps a few rehearsals would be in order. If he/she doesn't take the hint then I would advise you to take them aside and speak more directly. If they don't adapt then replace that person or do it yourself.
In defense of the pianist, at times a congregation can become 'stuck in their ways' and although it's our job to serve through music, often a little change or a new approach can be revitalizing, without becoming too extreme. Making larger, more drastic changes, to the worship music should be seen as a gradual and long-term renovation. More than often the shocking, sudden approach is not accepted by the majority. But with a little tact you can introduce some sustainable changes for the better. Perhaps the pianist felt the music was being 'dragged' too much and tried to pull the music along by playing fast. It would be more effective to speed it up gradually over a course of weeks, until the congregation grow accustomed to it. Often the congregation CAN sing a little faster but they decide they WON'T. This may be the situation here- i'm not sure.
But just be holistic when you are reviewing the situation. Consider everyone's perspective, carefully, before voicing your concerns. Be wise and fair. Maintain the harmony and happiness among the team. What's the point of being there otherwise?!
I hope this is a little help to you and i'm sure that i've touched on things you have already considered. But I will say one thing that spoke volumes to me... people may forget what you have said but they will never forget how you made them feel. The church is supposed to be a network of support and building up of each other...and at all lengths let this be your central goal. In any case, you made a very good first step to ask for advice. I'm sure there will be more people who will contribute further and give you some varied and useful tips and advice.
Lv N