How do native speaker pronounce when two words are connected by the "s sound", with one at the end of a word These and another in the beginning of another word sad. Do they have to stop at the middle of the word to make the "s sound" for sad or they just skip over it?
These / sad....
or
These-ad....
Firstly, your two sounds are different. As snailplane explains (better than me) 'these' ends in a long 'z' sound, rather than 's'. In this instance there is enough difference that speakers don't need to take any extra action when navigating that combination, just following the normal speech pattern works.
When you are dealing with a combination involving a proper 's' sound at the end of one word and the beginning of the next (eg 'this sad'), however, then difficulties may arise. Essentially, it depends on the speaker as to the approach to take, but both your suggestions are likely to be followed. If the speaker feels that the run-on approach will likely cause confusion or difficulty in understanding, then (s)he should enunciate the pause and ensure the listeners know that one word ends in 's' and the other begins with it. Similarly, a speaker would put more attention in if they were preparing a speech, rather than talking in conversation.
It isn't something that there is a hard and fast rule on, and can in fact cause confusion to native speakers. For evidence, check out the misheard lyrics site known as kissthisguy.com (which is a mishearing of 'kiss the sky').
Both are used. In normal conversation people tend to connect the s sound of both words, though they may spend a little more time on it than they would normally. If you're pausing for effect (or maybe to take a sip or something) then there can be a break. In more formal situations or in public speaking you might see people be very careful about their words and they might make two "s sounds" then.
