The adverb ταχύ· is variously translated as 'soon' or 'quickly' in Rev 3:11, according to Friberg when the adverb form is used in reference to time it means "without delay, right away, at once"1 Louw Nida says the same citing examples from other scriptures:
67.56 ταχύa; ταχέωςb; ταχινόςb, ή, όν; ἐν τάχει: pertaining to a point of time subsequent to another point of time (either an event in the
discourse or the time of the discourse itself), with emphasis upon the
relatively brief interval between the two points of time—‘soon, very
soon.’ ταχύa: οὐδεὶς γάρ ἐστιν ὃς ποιήσει δύναμιν ἐπὶ τῷ ὀνόματί μου
καὶ δυνήσεται ταχὺ κακολογῆσαί με ‘no one performing a miracle in my
name will be able soon afterward to say bad things about me’ Mk 9:39.
ταχέωςb: ἐλπίζω δὲ ἐν κυρίῳ Ἰησοῦ Τιμόθεον ταχέως πέμψαι ὑμῖν ‘I trust
in the Lord Jesus that I will be able to send Timothy to you soon’ Php
2:19. ταχινόςb: εἰδὼς ὅτι ταχινή ἐστιν ἡ ἀπόθεσις τοῦ σκηνώματός μου
‘I know that I shall soon put off this mortal body’ 2 Pe 1:14. ἐν
τάχει: δεῖξαι τοῖς δούλοις αὐτοῦ ἃ δεῖ γενέσθαι ἐν τάχει ‘to show his
servants what must happen soon’ Re 22:6.[Louw, J. P., & Nida, E. A.
(1996). Greek-English lexicon of the New Testament: based on semantic
domains (electronic ed. of the 2nd edition., Vol. 1, pp. 635–636). New
York: United Bible Societies.]
A footnote in the Baker Exegetical Commentary explains:
The emphasis in ταχύ is on imminence, not swiftness. This does not
mean Christ is coming “quickly” but “very soon” (as in 1:7; 22:7)
The point of the text therefore seems to be on the imminence of the Lord's return. It is close or near rather then the speed at which the Lord Jesus Christ is coming back as the 'quickly' might suggest in English.
1 [Fri] ταχύς, εῖα, ύ (1) as an adjective quick, prompt, ready (JA 1.19), opposite βραδύς (slow); (2) predominately, neuter singular ταχύ as an adverb; (a) as qualifying action quickly, swiftly, rapidly (MT 28.7); (b) as qualifying time without delay, right away, at once, soon (afterward) (MT 5.25); (3) comparative τάχιον; (a) more quickly, faster, sooner (JN 20.4); (b) without a time comparison quickly, without delay (JN 13.27); (4) superlative τάχιστα very quickly; with ὡς as soon as possible (AC 17.15)
ταχύς A--NM-S ταχύς