@tomchi007-
The SD card is only used to add additional channel app storage, not operating memory, and no matter how big a card you put in only 2 GB will be used.
The nature of streaming apps is that their developers keep updating and adding code to their apps which causes them to use up more of the capabilities of the devices they are being streamed on. Plus Roku keeps adding code to its operating system. Roku is rolling out Roku 13.0 right now which likely uses more resources than the 12.5 it replaces - has your Roku 3 been updated to it yet?
Your Roku 3 was extremely capable at the time of its release in 2013, but it may be reaching the point where it is underpowered and slow for running some of today's bloated apps. It may be time to update to a current more capable model. Roku 3s have 512 MB of memory, while today's offerings have faster processors, and with the exception of the bottom end non-4K Express, have 2 to 8 times the memory, and are faster and more responsive than a Roku 3.
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Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X