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It is possible for Twitter to have been trottling VPNs as they do collect location data and they want it to be acurate. More likely it just because when using a VPN your speeds are cut down about 60% and mobile devices tent to have slow connects already. So it is not surpising that app that require a lot of web traffic run into issues with VPNs.
The best solution for this would be for Twitter to be more lightweight and not require such a strong connection to run as the reason it needs such a strong connection is because it requires large amounts of information from the device.