I disagree. I run all engines and it's so light (even on a VM on an HDD) that I had to thoroughly test it to convince myself it was doing anything. I'll caveat that with I don't know how it will do on, like, a 9-year old C2D PC with 2GB RAM.
I did confirm (via my own tests and with tech support) that in a bottleneck (slamming Qihoo with ridiculous amount of requests), it will back off to not 1) hang or 2) interfere with performance. Qihoo designed their product to run on 9-year old PCs but kept it robust in execution and features. It's really an understated product.
I can't say anything about Baidu since they didn't have a Win10 version at the time of my choosing.