I found the performance of MT4 fell off a cliff, if I had more than 4 charts open at a time.
Might have something to do with processing power, something on the MT4 code, or my PC, ( as every other broker tried to convince me)
I suspected that the indicators I was using, were somehow resource hungry.
Nope!
MT5 is excellent.
I currently have 8 charts open, (dont know why, I only trade 2!). but performance, and latency is fine.
No glitchy mouse movements.
I now prefer MT5 over MT4.
Just my personal experience.
Also, if you use indicators at all, you might want to see where the development buzz is, MT4, or MT5, so you know if you will be served by the MT community.
Don't want to be in a position that add-ons that you may use, might not be supported, because everyones moved to MT5, etc.