This is assuming you have a standard tank/bowl toilet, and not one of those 'power flush' toilets that only has a small container concealed in what used to be the tank, that is designed to power water through the bowl with water pressure... This type of toilet DOES depend on native water (piped) pressure to function correctly. But this does not seem to be the problem you have.
A slow, or low pressure to a standard toilet will only affect the time it takes the toilet to cycle back to ready to flush.
Now you don't mention if it tries to back up, or if it just flushes slowly (as though the water is not flowing into the bowl fast enough). If it tries to back up, THEN flows out slow, then the problem is in the trap (the bend in the inner piping of the toilet that keeps sewer gasses out of your house) or in your sewer pipe, and may require a professional to clear. But since you only talk about it flushing slow, I'd figure it's a limiting clog somewhere between the tank and the bowl, and if you drain the tank, and carefully disassemble the toilet to clear or replace all the valves that regulate water flow between them, you'll probably see a notable change.
I hope this helps, and you have a great day!