Your laptop is probably slow for a reason, and it is very unlikely that the update would fix it.
It may be that the disk is almost full (which is sure to make any mac slower), or maybe there are software conflicts with stuff that is installed.
Please run Etrecheck and post its full report here.
Use the "additional text" button and paste the report into the text box.
By looking at the report we may get an idea of why it is so slow.
That is not to say that you shouldn't do the update - you should, but:
1) Make sure to have an up to date Time Machine backup
2) Before you update, you should address any issues arising from that report.