If being confident about Rotation Speed is important to you, investing in a strobe not controlled by the Mains Power will offer improved readings.
A dedicated Strobe such as the Key Strobe will give peace of mind.
I am not suggesting the following is something you are experiencing, it is made known to show how a dedicated strobe can be an additional value as a speed measure tool.
The strobe will also have the capability to show anomalies, where further investigation has for myself in the past, proven Platter Spindle Bearing Wear was present and a eccentric rotation was the result, the strobe was capable of showing the fluctuations in speed. A friend who specialises in refurbishing DD TT's has discovered many TT's with Platter Bearing Wear using this same method. For those who use his service, the option of having the Bearing rebuilt using modern materials, machined to extremely tight tolerances and bespoke produced suit each Spindle/Housing Interface. The end product is one where no more metal on metal is able to occur at the Spindle/Bush - Spindle/Ball-Thrust Pad interfaces.
This type of bearing mod' has become quite popular and for those that can swap out a bearing housing reasonably easy, there are many adopting this approach to utilising a modern bearing design.
DD TT's are a little more difficult, but this is a modification very much worthwhile investigating.