s following th is really hard for me. My tongue is never fast enough. I wonder if there is any reduction here. How do you pronounce it?
Native speakers of English typically do not reduce the th-s transition at all. The th sound, though famously difficult for people learning English as adults, usually poses no problem for people who learn to speak it as young children. To me it feels perfectly natural to go from th directly into s and I do it without a second thought.
If you have difficulty with th-s, you might just go ahead and pronounce "Smiths" as "Smits", which should be well enough understood by most people.
Here is an audio example.
When you make a "th" sound, your tongue should be in between your top and bottom teeth, touching at least your top row of teeth.
To then make the "s" sound, your tongue needs to slide from touching your teeth to touching the roof of your mouth, behind your top teeth.
For both "th" and "s" sounds you are blowing air out to creat the sound. Just practice moving your tongue from between your teeth back to the roof of your mouth while continuously breathing out the whole time.