[Performance] Grey screen is displayed way too long during switching between active devices

[rkatuzhanets]

STF version: 3.0
Platforms: iOS (Delay isn't so critical for Android)
Tested browsers: Chrome, Safari

Steps to reproduce:

• Open STF
• Grab 2+ devices across the platform (e.g. 2 iPhones)
• Complete some action on first device (e.g. open any website)
• Switch to another iOS device
• Accomplish some actions on this device
• Switch to back to the first device

Expected result:
Device's screen is reflected during commensurable with initial opening timeout (up to 5-7 seconds)

Actual result:
Device screen is in gray state after switching for a pretty protracted period of time (up to 30-40 seconds)