Google will try to detect your location and redirect you to a localized version of Google. For example, if you open google.com or google.co.uk in Thailand it will redirect you to google.co.th.
To stop this behavior, please visit google.com/ncr in your browser. (Once in each browser if you have multiple browsers.) You can now go to the Google version you prefer without it redirecting to the local version.
Google optimizes your experience by moving your Google data (e.g. your Gmail) to the nearest Google data-centre.
If you change locations frequently - VPN or otherwise - Google may not be able to do this adequately and slow down as a result. If possible, try to consistently use the same VPN server or VPN servers in the same area.