There are a few factors that can contribute to this. When you change the time frame of a chart that is using the TG_VSAScanner, it will not only have to load and recalculate the indicators on that chart, but all other charts that are set in the TG_VSAScanner settings. This could be up to 10 different time frames, it would be the equivalent of opening 10 new charts at the same time and waiting for them to calculate the studies.
For this reason, we strongly suggest that you do not change the timeframe of charts that have the scanner on them, instead, after the scanner is set on those charts, leave them alone and open a new chart for that symbol (currency pair, futures contract, etc...) and apply the included TradeGuider45 or TradeGuider47 template (the template will depend on the version of VSA you are using), or a custom template of your own that does not include the scanner.
The included TradeGuider45 and TradeGuider47 templates do not have the scanner, only the other indicators, and these will not affect the performance of the other charts that include the scanner. You can open as many charts as you like and add these templates or change the time frames on charts that have these templates without impacting the performance of the charts with the scanners.