Also strength depends on chess engine configuration. Many chess engines have such parameters as Threads (how many CPU cores use to calculate moves), Contempt (it's something like aggressiveness of play style), Hash size and many others. If you want to know more about it, here is good website: https://chessprogramming.wikispaces.com/
Always set Threads parameter to a max value in engine's settings (it is set to 1 by default). For example, if you have 4-core CPU, set Threads = 4, etc.
Use 64-bit versions of engines, instead of 32-bit versions, if possible (you need 64-bit Windows for that).
And there is one more important moment - time management. Chess engine can play by time or by fixed depth. Classic ChessBot supports only "play by depth" method (it has "floating depth" feature, but still). New ChessBotX supports "play by time" feature, please read more here about it.
Always use "play by time", when you need to achieve the strongest play.
do not use delays
in "bot vs bot" games. Also
do not use "Simulate human moves"
option. Also you can make ChessBot a little faster by experimenting with system delays.
Turn off animations at website settings if possible. Also try to decrease size of the chess board if possible (you can even try to lower your screen resolution for that). These recommendations will help the bot to react faster.