I want to do bonding in rb750r2 (two ports for wan). I will dial pppoe on it and have two ports in bridge as lan. How will it effect CPU ?

Answering the topic question: Yes, you can.

And how it will affect the CPU, depends on how much traffic you will be running on it, how many firewall rules it will have, etc.
Bonding feature just aggregate two or more network interfaces in a virtual interface and it’s controlled by CPU.

https://wiki.mikrotik.com/wiki/Manual: ... ng#Summary

I mean RB750r2 is a very tiny box with only 880 Mhz 1 core processor, and i have 15 filter rules, 15 nat rules, 10 mangle rules (because i'm running hotspot). The reason of asking this question is it has 100 Mb ports and by applying bonding i want to get 200 Mb. I mean 2 ports for wan and 2 ports for lan. I do not pass 200 Mb frequently but sometimes i have to transfer files so getting choked at 100 MB is a pain and CPU percentage reached at 100% so when i do bonding will the CPU adjust itself ?

I don't think that RB750r2 can route much more than 100Mbps with fasttrack disabled (implied by your use of mangle rules). And that's what you observed - 100% CPU when transferring at full speed. Bonding removes speed of single physical link as a bottleneck, it can't do anything about CPU load while device is routing.

Even setting that aside, bonding offers different modes, but only balance-rr allows that single connection utilizes both physical links, other modes limit single connection to single physical link (and balance-rr is not widely supported by switch vendors). Which means that bonding likely won't really help you to transfer files from one computer to another faster.