Apart from the old-fashioned and upper-class register, tush fits:

tush [interjection]

... used as an exclamation of impatience, disdain, contempt, etc.

[Dictionary.com]

• Tush, that's nothing, my friend did it in 15.

As these are at least really slang usages, another candidate is hmmpf / hmmf / ... (Wiktionary has the hmmpf variant):

Hmph (also hrmph or humph) indicates displeasure or indignation [including {depending on tone} a dismissive, scornful, perhaps even sneering riposte].

[DailyWritingTips]

• Hmmpf, that's nothing: my friend did it in 15.

I offer (see Lexico)

pfft
_EXCLAMATION

1.2 Used to express a contemptuous or dismissive attitude.

So you could remark

Pfft, that's nothing, my friend did it in 15.

p'shaw

used to express irritation, disapproval, contempt, or disbelief

-MW

If you like older expressions.

As in...

I made it in 30 minutes.

P'shaw, that's nothing, my friend did it in 15.

Otherwise:

henh?..

a fake laugh, usually used when someone says something obvious and stupid or not funny, or when someone says something over and over again to the point where it becomes stupid...

if you accept UD

As in:

I made it in 30 minutes.

Henh, that's nothing, my friend did it in 15.

"So what?" can be used to express how underwhelmed you are by a particular statement.

"Big deal" can be said sarcastically to indicate that the statement is not, in fact, a big deal at all.

1. You can almost always interject an obscenity of the sexual or scatological kind, but since that's a given it's almost redundant to mention here.

2. You can simply say "but" to indicate that you are contradicting the claim of extrordinarity: "But that's nothing!" Of course that may not strictly count as an interjection.

3. You can say "why", which, as Merriam-Webster notes under 4., is "used to express mild surprise, hesitation, approval, disapproval, or impatience", at least one of which should fit here: "Why, that's nothing, one of these newfangled auto-mobiles can make it easily in 10!" (Somehow I see the speaker in the England of the early 20th century...)

Sounds like a perfect situation for "meh". I've seen it mainly used as an interjection:

used to express indifference or mild disappointment

(Merriam-Webster).

I would say that it expresses "dismisiveness".

Note that you can also use it adjectively:

not impressive : so-so

(Same Merriam-Webster entry).

In my experience I see "meh" used more on the internet that in face-to-face conversation. On some internet social spaces "meh" seemed to get really popular in the 2010s, and it was common to see a response to a comment that looked like

Meh. The games for the PS4 are mostly knock-offs, and I .....

Be warned that the person going around saying "meh" to other people's remarks is going to sound like a bit of a jerk, but that seems to fit with the example conversation you've given.

(Personally, I've a adopted a policy of skipping any remarks that are prefaced with "meh", as I've found it too be a pretty reliable indicator of someone who doesn't show much respect for others. I'll be curious to see what other's attitudes towards "meh" are.)

The expressive interjection 'pah' goes to the heart of "disdain":

A. int.
 Expressing disgust or disdain.

OED, pah, int. and adj.

OED puts 'pah' in "Frequency Band 3", which they describe as containing

words which occur between 0.01 and 0.1 times per million words in typical modern English usage. These words are not commonly found in general text types like novels and newspapers, but at the same [time] they are not overly opaque or obscure.

eh

is an expression I've read a number of times in books.

In person it would frequently be accompanied by a shoulder shrug and possible a one eyebrow raise.